Heater, ventilation and air conditioning (HVAC) systems for automotive systems require fresh outside air to be drawn into the systems and incorporate blowers for air induction and distribution into the vehicle passenger compartment. The outside air sometimes includes dirt such as pollen, dust, debris or other contaminants which desirably is excluded from the passenger compartment. Accordingly a filter is interposed in the air stream at or just downstream of the intake opening, which is usually located in a space between the rear of the vehicle hood and the front edge of the windshield.
Air inlet filters gradually become clogged with dirt over an extended period of operation and thus should be replaced periodically with a clean filter element; failure to properly maintain the filter will eventually lead to a clogged condition. Where high humidity dampens the filter, dusty air can quickly clog the filter. In any event, when clogging occurs the system efficiency falls and normal air flow must be restored. For that purpose automatic filter retraction arrangements have been proposed.
Previous automatic filter retraction systems have used a filter which is movable from the air stream and is biased for such movement, a latch for holding the filter in operative position, and a sensor for determining when the filter is clogged and effective to release the latch, so that the filter will be moved to a position out of the air stream to allow unfiltered air to flow through the system. The system blower creates a suction on a filter which increases as dirt accumulates, so that the pressure differential across the filter can be used as an indicator of the filter condition. Prior systems have used diaphragms or other differential pressure sensors connected across the filter to release the latch While such sensors are effective, they are also expensive; it is thus desirable to accomplish the filter release without a separate sensor.